Exhaust emission control system for lean engines and method for operating the system

ABSTRACT

An emission control system for the cleaning of the exhaust gases of a lean burn engine with two or more cylinders comprises a first exhaust leg for the exhaust gases of a first group of cylinders and a second exhaust leg for the exhaust gases of a second group of cylinders. A nitrogen oxide storage catalyst is arranged in each exhaust leg. The two exhaust legs are combined downstream of the storage catalysts at a confluence to form a common exhaust leg. The common exhaust leg contains an SCR catalyst. The first and second groups of cylinders are each supplied alternately in periodic intervals with lean and rich air/fuel mixtures. Lean or rich exhaust gases are thus obtained in the combustion in the cylinders and released into the corresponding exhaust legs. Lean and rich exhaust gases are adjusted with respect to one another so as to result in a lean exhaust gas after the combination of the exhaust gases in the common exhaust leg. The regeneration of the storage catalysts may result in the formation of ammonia, which is stored by the SCR catalyst and reacted with nitrogen oxides which pass through the storage catalysts in an unwanted manner during the storage phases.

The invention relates to an emission control system for the cleaning of the exhaust gases of lean burn engines with two or more cylinders and to a method for operating the system. In the context of this invention, lean burn engines refer to diesel engines and lean burn petrol engines.

WO 2004/020807 A1 describes a two-flow emission control system for a diesel engine with a plurality of cylinders. The emission control system comprises a first exhaust leg for the exhaust gases of a first group of cylinders and a second exhaust leg for the exhaust gases of a second group of cylinders. A nitrogen oxide storage catalyst is arranged in each exhaust leg. The two exhaust legs are combined downstream of the storage catalysts at a confluence to form a common exhaust leg. The common exhaust leg contains an oxidation catalyst. The compositions of the exhaust gases in the first and second exhaust legs are adjusted independently by the electronic engine control system, such that the exhaust gas is enriched in one leg for regeneration of the storage catalyst, while the exhaust gas in the other leg is lean. Enrichment and depletion are adjusted such that a lean exhaust gas is present after the combination of the two exhaust gas streams in the common exhaust leg, and any possible slippage of the reducing agent is oxidized over the oxidation catalyst.

This emission control system has a crucial drawback: during the regeneration of a nitrogen oxide storage catalyst, considerable amounts of ammonia can be generated when the storage catalyst is regenerated for longer than necessary. This risk exists particularly in the case of aged storage catalysts. The ammonia formed flows together with the other exhaust gases over the oxidation catalyst in the combined exhaust leg and is oxidized again to nitrogen oxides, which reduces the control performance of the exhaust gas system with regard to the nitrogen oxide. This is the case especially at relatively high exhaust gas temperatures in the case of oxidation catalysts with high oxidation activity. Such catalysts are used in order to oxidize the hydrocarbons and carbon monoxide present in the exhaust gas at as early as possible a stage at low exhaust gas temperatures, for example after a cold start.

U.S. Pat. No. 6,047,542 describes an exhaust gas system for an engine which possesses a first and a second cylinder group. The first cylinder group is connected to a three-way catalyst.

The second group of cylinders and the three-way catalytist are connected via a common exhaust leg to an ammonia-absorbing and -oxidizing catalyst. When the first group of cylinders is operated under rich conditions, the second group of cylinders is operated under lean conditions. The three-way catalytic converter converts the nitrogen oxides present in the rich exhaust gas of the first cylinder group to ammonia, which reduces the nitrogen oxides emitted by the second cylinder group over the ammonia-absorbing and -oxidizing catalyst in the common exhaust leg. A nitrogen oxide storage catalyst inserted in the exhaust leg between the second cylinder group and the common exhaust leg reduces the amount of nitrogen oxide which flows into the ammonia-absorbing and -oxidizing catalyst.

WO 2006/008625 describes an exhaust gas treatment system for a lean burn engine with an SCR reactor downstream of an NOx adsorber (nitrogen oxide adsorber). The nitrogen oxide adsorber is regenerated with syngas from a fuel reforming reactor. The nitrogen oxide adsorber preferably possesses a catalytic function for converting the nitrogen oxides during the regeneration. The SCR reactor increases the conversion of the nitrogen oxides by storing the ammonia formed during the regeneration and using the stored ammonia to convert the nitrogen oxides during the lean operating mode of the engine. To reduce slip of ammonia, an oxidation catalyst is arranged downstream of the SCR reactor.

WO 2004/090296 discloses, in FIG. 1, a single-line exhaust gas aftertreatment unit. It comprises, in the exhaust gas flow direction, downstream of an internal combustion engine, in the full flow of the exhaust gas line, in succession, a reforming unit which simultaneously acts as a particulate filter, a nitrogen oxide storage catalyst and an SCR catalyst as emission control components. In the reforming unit, hydrogen is obtained by steam reforming, partial oxidation of hydrocarbons and/or mixed forms thereof.

U.S. Pat. No. 6,732,507 B1 likewise describes a single-line nitrogen oxide aftertreatment system in which a nitrogen oxide adsorber is combined with an SCR catalyst. The nitrogen oxide aftertreatment system is operated alternately with rich and lean air/fuel mixtures. The SCR catalyst stores the ammonia generated by the nitrogen oxide absorber during the regeneration in the rich exhaust gas and converts, with the ammonia stored, the nitrogen oxides which have not been adsorbed during the lean operation of the nitrogen oxide adsorber to harmless products. The SCR catalyst possesses a first end which is directly connected to the second end of the nitrogen oxide adsorber.

It is an object of the present invention to modify the known exhaust gas system of WO 2004/020807 A1 in such a way that the ammonia formed in an unwanted manner in the regeneration of the storage catalysts is neither oxidized to nitrogen oxides nor released to the environment. Furthermore, the slip of reducing agents during the regeneration should also be rendered harmless.

This object is achieved by the exhaust gas system according to the main claim. Claim 5 describes a method for operating the exhaust gas system.

The invention relates to lean burn engines with at least two cylinders. The lean burn engines preferably have four, six or more cylinders which are combined in a first group and in a second group of cylinders, each of which can be supplied independently with an air/fuel mixture.

The engines may be configured as in-line engines in which all cylinders are arranged in series in a single cylinder bank. Alternatively, each group of cylinders may be combined in a separate cylinder bank.

The emission control system of these lean burn engines comprises a first exhaust leg for the exhaust gases of the first group of cylinders and a second exhaust leg for the exhaust gases of the second group of cylinders. Each exhaust leg contains at least one nitrogen oxide storage catalyst. The two exhaust legs are combined downstream of the storage catalysts at a confluence to form a common exhaust leg. This emission control system is characterized in that the common exhaust leg contains an SCR catalyst.

The invention utilizes the storage action of SCR catalysts for ammonia in order to store any ammonia formed in the regeneration of the nitrogen oxide storage catalysts.

When the nitrogen oxides are stored in the nitrogen oxide storage catalysts and the storage catalysts are regenerated, there may be unwanted slip of nitrogen oxides during the storage phase. This slip of nitrogen oxides can be converted to nitrogen with the ammonia stored by the SCR catalyst. Moreover, an SCR catalyst has sufficient oxidation activity in order to convert any slip of reducing agents (hydrocarbons, carbon monoxide and hydrogen) with the oxygen content of the exhaust gases to harmless components.

In the context of this invention, a nitrogen oxide storage catalyst is understood to mean a catalyst which oxidizes the nitrogen monoxide present in a lean exhaust gas to nitrogen dioxide during a storage phase and then stores it in the form of nitrates. The mode of operation of nitrogen oxide storage catalysts is described in detail in the SAE document SAE 950809. For oxidation of nitrogen monoxide, a storage catalyst usually contains platinum, with or without palladium, as catalytically active components. The materials used for storage of the nitrogen oxides as nitrates include basic oxides, carbonates or hydroxides of alkali metals, alkaline earth metals and rare earth metals; preference is given to using basic compounds of barium and of strontium.

After exhaustion of its storage capacity, a storage catalyst has to be regenerated during a regeneration phase. To this end, the exhaust gas is briefly enriched, for example by operating the engine with a rich air/fuel mixture. In the rich exhaust gas, the nitrogen oxides are desorbed again and reduced to nitrogen with the aid of the rich exhaust gas constituents over the catalytically active components. For this purpose, the storage catalyst usually contains rhodium in addition to platinum.

During the regeneration, the rich constituents of the exhaust gas are converted to harmless components in an exothermic reaction with the nitrogen oxides stored in the catalyst and with any oxygen stored and residual oxygen still present. This heats the exhaust gas during the regeneration over the storage catalyst. The exhaust gas is heated additionally by virtue of the fact that the air content in the cylinder is greatly reduced by throttling of the engine during the rich operation and hence the exhaust gas is not cooled by high air excess as in lean operation. The two effects together can lead to a temperature increase in the exhaust gas over the storage catalyst of 50 to 150° C. during the regeneration.

Storage phase and regeneration phase alternate regularly. The storage phase usually lasts between 60 and 120 seconds, whereas the duration of the regeneration phase is only between 1 and 10% of that of the storage phase and hence is only a few seconds. The short regeneration time increases the risk that the storage catalyst regenerates for longer than required, i.e. is supplied with rich exhaust gas. Under these conditions, the storage catalyst forms ammonia from the nitrogen oxides.

Oxidation catalysts refer here to those catalysts which oxidize hydrocarbons and carbon monoxide to carbon dioxide and water in the lean exhaust gas. For this purpose, oxidation catalysts comprise, as the catalytically active component, platinum with or without palladium. These oxidation catalysts also oxidize ammonia to nitrogen and nitrogen oxides.

SCR catalysts are understood to mean catalysts which convert nitrogen oxides selectively to nitrogen under lean exhaust gas conditions with addition of ammonia as a reducing agent. These catalysts contain acidic oxides and can store ammonia. Typical SCR catalysts contain, for example, vanadium oxide and/or tungsten oxide on titanium oxide. Alternatively, it is also possible to use copper- and/or iron-exchanged zeolites. Usually, such catalysts do not contain any catalytically active platinum group metals since these metals would oxidize the ammonia in the lean exhaust gas to nitrogen oxides. For the inventive emission control system, preference is given to using SCR catalysts which comprise zeolites. Zeolites have a particularly high storage capacity for ammonia and for hydrocarbons. They are therefore outstandingly suitable for the storage and conversion of these components of the exhaust gas with nitrogen oxides.

The storage effect of the SCR catalysts for ammonia depends very greatly on the temperature. In particular after ageing of the catalysts in real operation, the storage effect declines very greatly above 300° C. and is barely perceptible any longer at temperatures above 400° C. Therefore, particularly at high exhaust gas temperatures, there is the risk that too much ammonia metered in leaves the SCR catalyst with the exhaust gas before it can react with the nitrogen oxides. In order to prevent this, a so-called ammonia barrier catalyst is usually arranged downstream of the SCR catalyst. In the simplest case, this is an oxidation catalyst which, however, can also reoxidize the ammonia to nitrogen oxides under unfavourable operating conditions.

The nitrogen oxide storage catalysts, oxidation catalysts and SCR catalysts used in the context of this invention are known to those skilled in the art. The catalysts are preferably applied in the form of a coating to inert honeycombs of ceramic or metal.

It is an advantage of the SCR catalyst in the common exhaust leg that it barely, if at all, oxidizes any nitrogen monoxide which breaks through to nitrogen dioxide—in contrast to the oxidation catalyst. This property is particularly important with regard to the expected emissions legislation for the emission of nitrogen dioxide. Nitrogen monoxide harms the environment to a lesser degree than nitrogen dioxide.

A further advantage of the inventive emission control system is the fact that the SCR catalyst, caused by the design of the system, has a large distance between the nitrogen oxide storage catalysts and the SCR catalyst. The exhaust leg between the storage catalysts and the SCR catalyst may be 0.5 to 1.5 metres. In the course of flow through this exhaust leg, the exhaust gas cools down by about 50° C. per metre of exhaust leg. A further crucial advantage of the process according to the invention is that storage and regeneration phases of the two cylinder groups are offset in time with respect to one another, as a result of which the exhaust gas in the common exhaust line has temperature variations of lesser magnitude and lower maximum temperatures in storage/regeneration operation than would be the case directly downstream of the nitrogen oxide storage catalysts in the individual exhaust lines. This leads to the effect that the SCR catalyst, over wide operating ranges of the engine, has a temperature at which the catalyst has a high storage effect for ammonia and is thus capable of converting the nitrogen oxides which break through the storage catalysts during the lean phase to harmless products with the ammonia stored.

In a specific embodiment of the invention, there is an oxidation catalyst downstream of the SCR catalyst in the common exhaust leg. SCR catalyst and oxidation catalyst may be arranged in series in separate housings. In this arrangement, the exhaust gas must heat the two separate catalysts to operating temperature. This is additionally complicated by heat losses between the two catalysts. It is therefore preferred for thermal reasons to apply both catalysts in the form of coatings to a common honeycomb as a support of the coatings. This combined SCR and oxidation catalyst may be designed as a so-called zone catalyst, which means that the SCR catalyst is applied on an inflow part of the honeycomb and the oxidation catalyst on an outflow part of the honeycomb.

It is particularly preferred, however, to apply the oxidation catalyst in the form of a first layer to a honeycomb and to apply the SCR catalyst as a second layer to this first layer. This arrangement has an outstanding barrier effect for ammonia and additionally also converts residual nitrogen oxides.

In further embodiments of the invention, the nitrogen oxide storage catalysts may be connected upstream of oxidation catalysts or three-way catalysts, for example in a position close to the engine, in order to reduce cold start emissions and to promote the oxidation of nitrogen monoxide to nitrogen dioxide in normal operation. Combination with a diesel particulate filter is also possible.

According to the invention, the emission control system described here is operated as follows: lean exhaust gas flows through the two nitrogen oxide storage catalysts during a storage phase, and rich exhaust gas during a regeneration phase, storage phase and regeneration phase alternating cyclically. The regeneration phase of one of the two storage catalysts is initiated whenever the other storage catalyst is in its storage phase. Lean and rich exhaust gas are adjusted with respect to one another so as to result in a lean exhaust gas after the combination of the exhaust gases in the combined exhaust leg.

Lean and rich exhaust gases are preferably obtained by operating the cylinders assigned to the two storage catalysts with lean or rich air/fuel mixtures and releasing them into the corresponding exhaust legs.

Alternatively, the engine can also be operated constantly with lean air/fuel mixture. In this case, the exhaust gas in the two exhaust legs is enriched in each case by injecting reducing agents for regeneration of the storage catalysts. Suitable reducing agents are, for example, fuel or other hydrocarbons. This mode of operation may be advantageous particularly in diesel engines.

The correct running of these operations is preferably monitored by an electronic engine control system. This engine control system regulates the compositions of the exhaust gases in the two exhaust legs independently of one another. It supplies, for example, the first group of cylinders assigned to the first exhaust leg with lean air/fuel mixture during the storage phase and initiates the regeneration of the nitrogen oxide storage catalyst in the second exhaust leg during this phase by briefly supplying the second group of cylinders assigned to the second exhaust leg with rich air/fuel mixture. This operation is repeated periodically in the reverse sequence in each case.

The invention is illustrated in detail by FIGS. 1 to 5. The figures show:

FIG. 1: Emission control system according to the invention with an SCR catalyst in the common exhaust leg

FIG. 2: Emission control system according to the invention with an SCR catalyst in the common exhaust leg and an oxidation catalyst arranged downstream thereof

FIG. 3: Emission control system according to the invention with an SCR catalyst and an oxidation catalyst on a honeycomb in the common exhaust leg

FIG. 4: Emission control system according to the invention with a combination catalyst composed of a layer of an SCR catalyst atop a layer of an oxidation catalyst in the common exhaust leg

FIG. 5: Schematic diagram of the air ratios λ against time in the first and second exhaust leg and in the common exhaust leg

FIGS. 1 to 4 show four embodiments of the emission control system. The same reference numeral denotes components of the same type. Reference numeral (1) denotes a lean burn engine with two cylinder banks (2) and (2′). The exhaust gases of these cylinder banks are released into the two exhaust legs (3) and (3′). At the confluence (4), the two exhaust legs (3) and (3′) are combined to form a common exhaust leg (5). For storage and conversion of the nitrogen oxides emitted by the lean burn engine (1), the nitrogen oxide storage catalysts (6) and (6′) are arranged in the exhaust legs (3) and (3′).

According to the invention, an SCR catalyst (7) is present in the common exhaust leg (5). It stores the unwanted ammonia which forms in the course of regeneration of the storage catalysts. The correct running of storage phase and regeneration phase is preferably monitored by an electronic engine control system. This engine control system and the necessary sensors for the determination of the air ratio in the two exhaust legs are not shown in the figures for the sake of simplicity. Electronic engine control systems and the necessary sensors for operation of a lean burn engine with nitrogen oxide storage catalysts are known to those skilled in the art. For the operation of the inventive exhaust gas system according to the method described, the control programme has to be adjusted correspondingly.

In a specific embodiment of the invention, as shown in FIG. 2, an oxidation catalyst (8) is inserted into the common exhaust leg downstream of the SCR catalyst (7).

FIG. 3 shows the design of the emission control system in a preferred embodiment of the invention. In the common exhaust leg (5), the SCR and oxidation catalysts are now arranged in direct succession. This combination of SCR and oxidation catalysts can be configured, for example, as zone catalyst on a single continuous honeycomb.

FIG. 4 shows a further embodiment of the invention. In this embodiment, the catalyst (9) in the common exhaust leg is designed as a combined SCR and oxidation catalyst. The catalyst has an oxidation catalyst as a first layer on an inert honeycomb. The SCR catalyst is applied as a second layer to this oxidation catalyst. This second layer is in direct contact with the exhaust gas.

FIG. 5 shows, in schematic form, the air ratio lambda (λ) against time in the first exhaust leg (curve a)) relative to that in the second exhaust leg (curve b)) and in the common exhaust leg (curve c)). The air ratio λ is the air/fuel ratio normalized to stoichiometric conditions.

The dotted reference line in FIG. 5 shows in each case the stoichiometric ratio λ=1. The storage phase with λ>1 (lean exhaust gas) alternates regularly with the regeneration phase with λ<1 in the two exhaust legs. The regeneration phase is significantly shorter than the storage phase. The phase position of the two lambda curves a) and b) relative to one another is substantially variable, provided that the exhaust gas in the combined exhaust leg is always lean (curve c)), i.e. has a lambda value greater than 1.

A particular advantage of the proposed emission control system and of the method for operation thereof is the fact that unwanted ammonia which forms in the regeneration of the storage catalysts is stored by the downstream SCR catalyst. The ammonia stored selectively converts nitrogen oxides which pass through the storage catalysts during the regeneration phase to nitrogen. This additionally reduces nitrogen oxide emission. 

1. Emission control system for the cleaning of the exhaust gases of a lean-burn engine with two or more cylinders, wherein a first exhaust leg accommodates the exhaust gases of a first group of cylinders and a second exhaust leg the exhaust gases of a second group of cylinders, and a nitrogen oxide storage catalyst is arranged in each exhaust leg and both exhaust legs are combined downstream of the storage catalysts at a confluence to form a common exhaust leg, characterized in that an SCR catalyst is present in the common exhaust leg.
 2. Emission control system according to claim 1, wherein, an oxidation catalyst is arranged downstream of the SCR catalyst in the common exhaust leg.
 3. Emission control system according to claim 2, wherein SCR catalyst and oxidation catalyst are applied to a common honeycomb, the SCR catalyst being arranged on the inflow part of the honeycomb and the oxidation catalyst on the outflow part of the honeycomb.
 4. Emission control system according to claim 1, wherein an oxidation catalyst is present on a honeycomb in the form of a first layer and the SCR catalyst is applied as a second layer to this first layer.
 5. Method of operating the emission control system according to claim 1, wherein lean exhaust gas flows through the two nitrogen oxide storage catalysts during a storage phase, and rich exhaust gas during a regeneration phase, and storage phase and regeneration phase alternate cyclically, the regeneration phase of one of the two storage catalysts being initiated whenever the other storage catalyst is in its storage phase, and lean and rich exhaust gas being adjusted relative to one another so as to result in a lean exhaust gas after the exhaust gases have been combined in the common exhaust leg, and this exhaust gas being passed over an SCR catalyst.
 6. Method according to claim 5, wherein lean and rich exhaust gases are obtained by operating the cylinders assigned to the two storage catalysts with lean or rich air/fuel mixtures and released into the corresponding exhaust legs.
 7. Method according to claim 5, wherein the engine is operated constantly with a lean air/fuel mixture and the exhaust gas in the two exhaust legs is made richer in each case by injecting fuel or hydrocarbons for regeneration of the storage catalysts. 